The invention relates to the field of remote metering and more particularly to apparatus for remotely delivering the value of one or several parameters sensed on an apparatus or a machine to be tested or monitored.
Numerous multichannel telemetering systems exist which provide a remote indication or recording of several parameters taken by means of sensors. On the other hand, these systems include a complex measuring head, which is bulky and whose inplantation on a machine often necessitates considerable study and the construction of laborious parts. That occurs for telemetering on rotary machines, where the parts for supporting the head must be attached without modifying the shaft of the machine to a point reducing its mechanical strength, where available space is limited and where however it is often necessary to resist to acceleration. In addition, the electrical supply of such heads requires the presence of additional lines which further complicate the problem, all the more as the head frequently includes an analogue portion and a digital portion which render several supplies necessary and leads to the use of tori for the construction of electric supply transformers. The presence of a considerable magnetic field in electrical rotary machines, results in saturation of these tori and loss of energy supply.
Very often, the possibilities offered by a multichannel telemetering system are unnecessary and a single-channel apparatus would be sufficient to meet the needs, for example for interventions to be carried out rapidly, which only require measurement of a single parameter.
It is an object of the invention to provide a single-channel measuring head whose measurement accuracy is comparable with that of existing complex multichannel heads, which is simple and of low bulk, which has low requirements as regards power supply and which is easily located.
According to the invention, there is provided a single-channel telemetering head comprising a sensor supply, amplification and filtering means, a converter with a duration modulation and transmission of high and low references, enabling the value of the signal received by the converter on reception to be reconstituted, and transmission means.
The low consumption of such a head, which can be produced from analogue components with a low consumption and logic circuits with C-MOS transistors, permits it to operate from a single power source of low voltage and capacity, which may be a battery or an accumulator, without the use of tori sensitve to high electromagnetic fields. The very low consumption gives the head a great self-sufficiency. The assembly, having a low number of components, can be miniaturized and its insertion inside a machine is done at low price, conveniently and rapidly.
The converter comprises advantageously a ramp generator and triggering comparators for delivering square pulses when the ramp reaches three values respectively corresponding to low and high references which are generated locally and to the measured value. The measured value is then represented, after conversion, by a time interval which separates the square pulses representing the low reference and representing occurrence of the measured value. This time interval must be related to the time interval which separates the square pulses representing the passage to the low and high reference values. Any modification of the interval between crossing of the two reference values by the ramp reflect changes of the system and permit, by simple treatment by a receiving unit, to determine corrections to be effected, for example by modifying the charging current of the capacitor generally used in the ramp generator, as a function of temperature.
The square pulses are generally created by monostable devices placed downstream of the comparators, the set or hold period of one of the monostable devices being different from the others so as to enable identification of the square waves by means of a logic circuit included in the receiving unit.
The invention also proposes an apparatus comprising a head of the above-defined type and a receiving unit having a ramp generator or digital restitution converter, actuated by the square pulses coming from the head.
The invention will be better understood on reading the description which follows of a particular embodiment, given by way of example.